MODULE 2 MATTER IN MOTION

1A NEWTON’S LAWS OF MOTION

1. In pairs or small groups, discuss the questions:

1. What exactly is speed?

2. What is velocity?

3. What is acceleration?

2. Do you remember Newton’s laws of motion?

Every action of a force produces an equal and opposite reaction.

Bodies move in a straight line with a uniform speed, or remain stationary, unless a force acts to change their speed or direction.

Forces produce accelerations that are in proportion to the mass of a body (F= ma)

3. Match the terms with their definitions

4. Read and complete the text using the words below.

NEWTON’S LAWS of MOTION

Isaac Newton was one of the most prominent, contentious and influential scientists of all time. He helped to invent calculus, explained gravity and identified the constituent colors of white light. His three laws of motion describe why a golf ball follows a 1)… path, why we are pressed against the side of a cornering and why we feel the force through a baseball bat as it strikes the ball.

Borrowing Galileo’s principle of inertia, Newton formulated his fist law. It states that bodies do not move or 2)… their speed unless a force acts. Bodies that are not moving will remain stationary unless a 3)… is applied; bodies that are moving with some constant speed keep moving at that same speed unless acted upon by a force. A force supplies acceleration that changes the velocity of the object. 4)… is a change in speed over some time.

This is hard to appreciate in our own experience. If we throw a hockey puck it skims along the ice but eventually slows due to friction with the ice. Friction causes a force that decelerates the puck. But Newton’s first law may be seen in a special case where there is no 5)… . The nearest we might get to this is in space, but even here there are forces such as gravity at work. Nevertheless, this first law provides a basic touchstone from which to understand forces and motion.

Newton’s second law of motion relates the size of the force to the acceleration it produces. The force needed to accelerate an object is proportional to the object’s mass. 6)… objects - or rather ones with large inertia – need more force to accelerate them than lighter objects. So to accelerate a car from standing still to 100 kilometers an hour in one minute would take a force equal to the car’s mass times its increase in speed per unit time. Newton’s second law is expressed algebraically as ‘F= ma’, force (F) equals mass (m) times acceleration (a). Turning this definition around, the second law expressed in another way says that acceleration is equal to force per unit mass. For a constant acceleration, force per unit mass is also unchanged. So the 7)… amount of force is needed to move a kilogram mass whether it is part of a small or large body. This explains Galileo’s imaginary experiment that asks which would hit the ground first if dropped together: a cannonball or a feather? Visualizing it we may think that the cannonball would arrive ahead of the drifting feather. But this is simply due to the air resistance(h) that wafts the feather. If there were no 8)… , then both would fall at the same rate, hitting the ground together. They experience the same acceleration, gravity, so they fall side by side. Apollo 15 astronauts showed in 1971 that on the Moon, where there is no atmosphere to slow it down, the feather falls at the same rate as a geologist’s heavy hammer.

Newton’s third law states that any force applied to a body produces an equal and opposite reaction force in that body. In other words, for every action there is a reaction. The opposing force is felt as recoil. A marksman feels the kick of the rifle against his shoulder as he shoots. The recoil force is equal in size to that originally expressed in the shove or the bullet. In crime films the victim of a shooting often gets propelled 9)… by the force of the bullet. This is misleading. If the force was really so great then the shooter should also be hurled back by the recoil of his gun. Even if we jump off the ground, we exert a small downward force on the Earth, but because the Earth is so much massive than we are, it barely shows.

With these three laws, plus 10)… , Newton could explain the motion of practically all objects. Armed with these three equations he could confidently have climbed aboard a fast motorbike and sped up onto the wall of death, had such a thing existed in his day. How much trust would you place in Newton’s laws? The first law says that the cycle and its rider want to keep travelling in one direction at a certain speed. But to keep the cycle moving in a circle, according to the second law, a confining force needs to be provided to continually change its 11)… , in this case applied by the track through the wheels. The force needed is equal to the mass of the cycle and rider multiplied by their acceleration. The third law then explains the pressure exerted by the cycle on the track, as a reactionary force is set up. It is this pressure that glues the stunt rider to the inclined wall, and if the bike goes fast enough it can even ride on a vertical wall. Where Newton’s laws do not hold is for things moving close to the speed of light or with very small masses. It is in these extremes that Einstein’s relativity and the science of quantum mechanics take over.

Backwards heavy direction air gravity curving same friction change acceleration force

5. Read the text again and find the words that mean the same as the following phrases.

Little by little;

a. action of one object or surface moving against another;

b. to throw sth/sb violently in a particular direction;

c. a property of matter by which it stays still or if moving, continues moving in a straight line unless it is acted on by a force outside itself;

d. added a number to itself a particular number of times;

e. the force or weight with which sth pressed against sth else;

f. to move, or push sth forward or in a particular direction;

g. a sudden movement backwards, especially of a gun when it is fired;

h. a force that stops sth moving or makes it move more slowly;

i. to begin to have control of or responsibility for sth;

j to use power to affect sb/sth;

k repeated many times without interruption.

6. In pairs, discuss and write definitions for the highlighted terms from the text. Use a dictionary to help you.

7. Answer the questions to the text.

1. What does the first law of motion postulate? Where can we see the action of the law in practice?

2. What does the second law of motion state? How is it expressed algebraically? Can you give examples of its work?

3. What is the third law of motion? How does it work?

4. Where don’t Newton’s laws of motion work? Why? What theories explain the motion there?

8. Read the sentences and mark (T) True or (F) false.

1. There are no perfect demonstrations of the first law of motion , as friction causes a force to act on a moving body.

2. Newton’s second law states that for every action there is reaction.

3. Friction causes a force that decelerates the object.

4. The force needed to accelerate an object is proportional to the object’s size.

5. Newton’s laws still work for the objects moving close to the speed of light.

9. Complete these sentences with information that reflects your personal views.

1. There are many applications of Newton’s first law of motion…

2. The behavior of all objects can be described by saying that objects tend to ‘keep on doing what they’re doing’…

3. Newton’s laws are not applicable on non-inertial frames of reference…

10. In pairs, discuss the following diagram.

Forces are Balanced

↙ ↘

Objects at Rest Objects in Motion

(v = 0 m/s) (v≠0 m/s)

↓ ↓

a= 0 m/s ² a = 0 m/s²

↓ ↓

Stay at Rest Stay in Motion

11. Watch a video’ Newton’s laws of motion’ and do the task.

1. Describe the first experiment and explain it.

2. Tell what the acceleration of an object depends on.

3. Describe the second experiment and explain what happens if the mass has doubled.

4. What equation expresses the relationship between force, mass, and acceleration?

5. Tell how the third law of motion explains the flight of the rocket into space.

12. Check your understanding

1. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is the clear case of Newton’s third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?(Each force is the same size)

2. For years, space travel was believed to be impossible because there was nothing that rockets could push off of the space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because…

1. … space is void of air so the rockets have nothing to push off of.

2. …gravity is absent in space.

3. … space is void of air and so there is no air resistance in space.

4. ….nonsense! Rockets do accelerate in space and have been able to do so for a long time.

3. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand out –ward allowing the rifle to push forward on the bullet. Consistent with the Newton’s third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is…

1. … greater than the acceleration of the bullet.

2. … smaller than the acceleration of the bullet.

3. … the same size as the acceleration of the bullet.

13. In pairs, role-play a conversations in which a scientist explains to a lay person the significance of Newton’s laws of motion.

14. Translate the following paragraph into Russian.

A variety of action –reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.